Such method and a plant are known for instance from DE 44 10 093 C1, in order to reduce iron-oxide-containing solids such as iron ores, iron ore concentrates or the like. For this purpose, iron-oxide-containing ore is introduced into the fluidized-bed reactor and fluidized with heated reduction gas. The solids are entrained by the gas stream and separated from the exhaust gas in a downstream separator, in order to be recirculated to the reactor. For further processing, solids are withdrawn from the lower region of the reactor.
However, this direct reduction in the fluidized-bed reactor involves the risk that above the distributor grid (gas distributor), which is typically used for distributing the reduction gas, excess temperatures are obtained, as the reduction gas has a high temperature for heating the solids. At the same time, the mass and heat transfer conditions are regarded as unsatisfactory due to the only limited solids retention time of e.g. 20 minutes and the only moderate relative velocities between gas and solids. Due to the additional introduction of exhaust gases of a downstream reduction stage into the reactor, the reactor must have a complicated design, for instance with a diameter reduced by about 30% in the vicinity of the exhaust gas inlet. In addition, the gas supply conduits are gated at an angle, in order to prevent a clogging of the conduit to which dust-laden (secondary) gas is admitted, and to allow that solids which have entered the conduit fall back into the reactor.